In recent years, as growing importance is attached to resource saving and environmental protection, the demand for a reduction in the fuel consumption of automobiles has become increasingly stronger. Also for automobile tires, it is desired to reduce their rolling resistance and thereby contribute to a reduction in fuel consumption. In order to reduce the rolling resistance of tires, it is common practice to use, as the rubber material for tires, a rubber material which can yield a vulcanized rubber showing a low degree of heat build-up.
Conventionally, it has been proposed to reduce heat build-up by using, as the rubber material for tires, a rubber composition comprising a diene rubber into which, in place of carbon black, silica is incorporated as a reinforcing agent. However, as compared with carbon black-filled rubber compositions, such silica-filled rubber compositions have the disadvantage that they fail to achieve sufficient abrasion resistance and tensile strength. One of the causes therefor is believed to be that silica has a lower affinity for diene rubbers than carbon black and hence fails to exhibit a sufficient reinforcing effect.
Conventionally, in order to enhance the affinity of silica for diene rubbers, the use of a diene rubber into which a substituent group having an affinity for silica has been introduced is being investigated. For example, diene rubbers having a tertiary amino group introduced thereinto (Japanese Patent Laid-Open No. 101344/'89) have been proposed for diene rubbers formed by emulsion polymerization; and diene rubbers having introduced thereinto an alkylsilyl group (Japanese Patent Laid-Open No. 188501/'89), a halogenated silyl group (Japanese Patent Laid-Open No. 230286/'93) or a substituted amino group (Japanese Patent Laid-Open No. 22940/'89) have been proposed for diene rubbers formed by anionic polymerization.
However, most of the diene rubbers having the aforesaid substituent groups introduced thereinto show poor processability because, when they are mixed with silica, they cohere strongly with silica and cannot be dispersed satisfactorily. Moreover, they also have the disadvantage that vulcanized rubber properties such as heat build-up, tensile strength and abrasion resistance are not fully improved.
An object of the present invention is to provide a diene rubber which has excellent processability and, when silica is incorporated thereinto as a reinforcing agent, can yield a vulcanized rubber that shows a low degree of heat build-up and has substantially the same tensile strength and abrasion resistance as when carbon black is incorporated thereinto, as well as a process for preparing the same.
Another object of the present invention is to provide a rubber composition containing a diene rubber and a reinforcing agent, and capable of yielding a vulcanized rubber which shows improvements in heat build-up, tensile strength, abrasion resistance and processability.